Re: [RESEND PATCH v7 00/10] Small-sized THP for anonymous memory

[John Hubbard]

On 11/23/23 08:50, David Hildenbrand wrote:
> On 23.11.23 17:18, Matthew Wilcox wrote:
> > On Thu, Nov 23, 2023 at 05:05:37PM +0100, David Hildenbrand wrote:
> > > On 23.11.23 16:59, Matthew Wilcox wrote:
> > > > On Wed, Nov 22, 2023 at 04:29:40PM +0000, Ryan Roberts wrote:
> > > > > Note: I'm resending this at Andrew's suggestion due to having 
> > > > > originally sent
> > > > > it during LPC. I'm hoping its in a position where the feedback is 
> > > > > minor enough
> > > > > that I can rework in time for v6.8, but so far haven't had any.
> > > > >
> > > > > Hi All,
> > > > >
> > > > > This is v7 of a series to implement small-sized THP for anonymous 
> > > > > memory
> > > > > (previously called "large anonymous folios"). The objective of this 
> > > > > is to
> > > >
> > > > I'm still against small-sized THP.  We've now got people asking whether
> > > > the THP counters should be updated when dealing with large folios that
> > > > are smaller than PMD sized.  It's sowing confusion, and we should go
> > > > back to large anon folios as a name.
> > >
> > > I disagree.
> > >
> > > https://lore.kernel.org/all/65dbdf2a-9281-a3c3-b7e3-a79c5b60b357@xxxxxxxxxx/
> >
> > And yet:
> > https://lore.kernel.org/linux-mm/20231106193315.GB3661273@xxxxxxxxxxx/
> >
> > "This is a small THP so we don't account it as a THP, we only account
> > normal THPs as THPs" is a bizarre position to take.
> >
> > Not to mention that saying a foo is a small huge baz is just bizarre.
> > Am I a small giant?  Or just a large human?
>
> I like that analogy. Yet, "small giant" sounds "bigger" in some way IMHO ;)
>
> I'll note that "small-sized THP" is just a temporary feature name, it 
> won't be exposed as such to the user in sysfs etc. In a couple of years, 
> it will be forgotten.
>
> To me it makes sense: it's a hugepage (not a page) but smaller compared 
> to what we previously had. But again, there won't be a "small_thp" 
> toggle anywhere.
>
> Long-term it's simply going to be a THP. Quoting from my writeup:
>
> "Nowadays, when somebody says that they are using hugetlb huge pages, 
> the first question frequently is "which huge page size?". The same will
> happen with transparent huge pages I believe.".
>
>
> Regarding the accounting: as I said a couple of times, "AnonHugePages" 
> should have been called "AnonPmdMapped" or similar; that's what it 
> really is: as soon as a THP is PTE-mapped, it's not accounted there. But 
> we can't fix that I guess, unless we add some "world switch" for any 
> workloads that would care about a different accounting.
>
> So we're really only concerned about:
> * AnonHugePages
> * ShmemHugePages
> * FileHugePages


The v6 patchset had these counters:

/proc/vmstat:  nr_anon_thp_pte
/proc/meminfo: AnonHugePteMap

...which leads to another naming possibility: pte-thp, or pte-mapped-thp,
something along those lines.

pte-thp avoids the "small huge" complaint, at least.


thanks,
--
John Hubbard
NVIDIA